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          <h1 class="post-title" itemprop="name headline">Vue中的computed和watch区别</h1>
        

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        <p>以vue.js官方文档中获取全名的一个例子：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> vm = <span class="keyword">new</span> Vue(&#123;</span><br><span class="line">  el: <span class="string">&#x27;#demo&#x27;</span>,</span><br><span class="line">  data: &#123;</span><br><span class="line">    firstName: <span class="string">&#x27;Foo&#x27;</span>,</span><br><span class="line">    lastName: <span class="string">&#x27;Bar&#x27;</span>,</span><br><span class="line">    fullName: <span class="string">&#x27;Foo Bar&#x27;</span></span><br><span class="line">  &#125;,</span><br><span class="line">  watch: &#123;</span><br><span class="line">    firstName: <span class="function"><span class="keyword">function</span> (<span class="params">val</span>) </span>&#123;</span><br><span class="line">      <span class="built_in">this</span>.fullName = val + <span class="string">&#x27; &#x27;</span> + <span class="built_in">this</span>.lastName</span><br><span class="line">    &#125;,</span><br><span class="line">    lastName: <span class="function"><span class="keyword">function</span> (<span class="params">val</span>) </span>&#123;</span><br><span class="line">      <span class="built_in">this</span>.fullName = <span class="built_in">this</span>.firstName + <span class="string">&#x27; &#x27;</span> + val</span><br><span class="line">    &#125;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>

<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> vm = <span class="keyword">new</span> Vue(&#123;</span><br><span class="line">  el: <span class="string">&#x27;#demo&#x27;</span>,</span><br><span class="line">  data: &#123;</span><br><span class="line">    firstName: <span class="string">&#x27;Foo&#x27;</span>,</span><br><span class="line">    lastName: <span class="string">&#x27;Bar&#x27;</span></span><br><span class="line">  &#125;,</span><br><span class="line">  computed: &#123;</span><br><span class="line">    fullName: <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">      <span class="keyword">return</span> <span class="built_in">this</span>.firstName + <span class="string">&#x27; &#x27;</span> + <span class="built_in">this</span>.lastName</span><br><span class="line">    &#125;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>

<h2 id="1、定义"><a href="#1、定义" class="headerlink" title="1、定义"></a>1、定义</h2><p><code>computed：</code>是一个计算属性,类似于过滤器,对绑定到view的数据进行处理</p>
<p><code>watch：</code>是一个观察的动作</p>
<h2 id="2、区别"><a href="#2、区别" class="headerlink" title="2、区别"></a>2、区别</h2><h3 id="2-1、特性"><a href="#2-1、特性" class="headerlink" title="2.1、特性"></a>2.1、特性</h3><p><code>computed：</code></p>
<p>具有缓存性，页面重新渲染值不变化,计算属性会立即返回之前的计算结果，而不必再次执行函数</p>
<p><code>watch：</code></p>
<p>无缓存性，页面重新渲染时值不变化也会执行</p>
<h3 id="2-2、在写法上有没有什么区别？"><a href="#2-2、在写法上有没有什么区别？" class="headerlink" title="2.2、在写法上有没有什么区别？"></a>2.2、在写法上有没有什么区别？</h3><p>写法上有一个区别，<code>computed</code><strong>返回</strong>state处理后结果，<code>watch</code>是赋值行为，<strong>修改</strong>state。</p>
<ul>
<li>注意事项：<strong>不应该使用箭头函数来定义 watcher 函数</strong>，因为箭头函数没有 this，它的 this 会继承它的父级函数，但是它的父级函数是 window，导致箭头函数的 this 指向 window，而不是 Vue 实例</li>
</ul>
<h3 id="2-3、哪个更加底层？"><a href="#2-3、哪个更加底层？" class="headerlink" title="2.3、哪个更加底层？"></a>2.3、哪个更加底层？</h3><p><code>watch</code>和<code>computed</code>并没有哪个更底层，<code>watch</code>内部调用的是<code>vm.$watch</code>，它们的共同之处就是每个定义的属性都单独建立了一个<code>Watcher</code>对象。</p>
<h3 id="2-4、常见应用有什么？"><a href="#2-4、常见应用有什么？" class="headerlink" title="2.4、常见应用有什么？"></a>2.4、常见应用有什么？</h3><ul>
<li><p><code>computed</code>就是简化tempalte里面Mustache语法计算和处理props或$emit的传值.</p>
</li>
<li><p><code>watch</code>监听props，$emit或本组件的值执行异步操作。</p>
</li>
</ul>
<p><code>computed</code>的值在<code>getter</code>执行后是会缓存的，只有在它依赖的属性值改变之后，下一次获取<code>computed</code>的值时才会重新调用对应的<code>getter</code>来计算。<code>watch</code>在每次监听的值变化时，都会执行回调。其实从这一点来看，都是在依赖的值变化之后，去执行回调。很多功能本来就很多属性都可以用，只不过有更适合的。</p>
<p>如果一个值依赖多个属性（多对一），用<code>computed</code>肯定是更加方便的。如果一个值变化后会引起一系列操作，或者一个值变化会引起一系列值的变化（一对多），用<code>watch</code>更加方便一些。<br><code>watch</code>的回调里面会传入监听属性的新旧值，通过这两个值可以做一些特定的操作。<code>computed</code>通常就是简单的计算。</p>
<h2 id="3、我该选择哪个？"><a href="#3、我该选择哪个？" class="headerlink" title="3、我该选择哪个？"></a>3、我该选择哪个？</h2><p><code>computed</code>是用于定义<strong>基于数据之上的数据</strong>。</p>
<p>而<code>watch</code>是你想在<strong>某个数据变化时做一些事情</strong>，如果做的事情是更新其他数据，那其实与把这个要更新的数据项定义成<code>computed</code>是一样的，这个时候用computed更有可读性，虽然技术上讲<code>watch</code>也可以实现。</p>
<p>但你也可以在被watch的数据变化时做其他事情，比如调用一个方法，这个是<code>computed</code>做不到也不应该做的。</p>
<p>正如文档建议说 执行异步操作或开销较大的操作时使用<code>watch</code>。</p>
<h3 id="3-1、总结"><a href="#3-1、总结" class="headerlink" title="3.1、总结"></a>3.1、总结</h3><ol>
<li>简单且串行返回的，使用<code>computed</code>。</li>
<li>如果一个数据依赖于其他数据，那么把这个数据设计为<code>computed</code>的</li>
<li>如果你需要在某个数据变化时做一些事情，使用<code>watch</code>来观察这个数据变化。</li>
<li>执行异步操作不能串行返回结果，使用<code>watch</code>。</li>
<li>开销较大的操作，避免堵塞主线程，使用<code>watch</code>。</li>
</ol>

      
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  <script type="text/javascript">
    // Popup Window;
    var isfetched = false;
    var isXml = true;
    // Search DB path;
    var search_path = "search.xml";
    if (search_path.length === 0) {
      search_path = "search.xml";
    } else if (/json$/i.test(search_path)) {
      isXml = false;
    }
    var path = "/" + search_path;
    // monitor main search box;

    var onPopupClose = function (e) {
      $('.popup').hide();
      $('#local-search-input').val('');
      $('.search-result-list').remove();
      $('#no-result').remove();
      $(".local-search-pop-overlay").remove();
      $('body').css('overflow', '');
    }

    function proceedsearch() {
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay"></div>')
        .css('overflow', 'hidden');
      $('.search-popup-overlay').click(onPopupClose);
      $('.popup').toggle();
      var $localSearchInput = $('#local-search-input');
      $localSearchInput.attr("autocapitalize", "none");
      $localSearchInput.attr("autocorrect", "off");
      $localSearchInput.focus();
    }

    // search function;
    var searchFunc = function(path, search_id, content_id) {
      'use strict';

      // start loading animation
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay">' +
          '<div id="search-loading-icon">' +
          '<i class="fa fa-spinner fa-pulse fa-5x fa-fw"></i>' +
          '</div>' +
          '</div>')
        .css('overflow', 'hidden');
      $("#search-loading-icon").css('margin', '20% auto 0 auto').css('text-align', 'center');

      $.ajax({
        url: path,
        dataType: isXml ? "xml" : "json",
        async: true,
        success: function(res) {
          // get the contents from search data
          isfetched = true;
          $('.popup').detach().appendTo('.header-inner');
          var datas = isXml ? $("entry", res).map(function() {
            return {
              title: $("title", this).text(),
              content: $("content",this).text(),
              url: $("url" , this).text()
            };
          }).get() : res;
          var input = document.getElementById(search_id);
          var resultContent = document.getElementById(content_id);
          var inputEventFunction = function() {
            var searchText = input.value.trim().toLowerCase();
            var keywords = searchText.split(/[\s\-]+/);
            if (keywords.length > 1) {
              keywords.push(searchText);
            }
            var resultItems = [];
            if (searchText.length > 0) {
              // perform local searching
              datas.forEach(function(data) {
                var isMatch = false;
                var hitCount = 0;
                var searchTextCount = 0;
                var title = data.title.trim();
                var titleInLowerCase = title.toLowerCase();
                var content = data.content.trim().replace(/<[^>]+>/g,"");
                var contentInLowerCase = content.toLowerCase();
                var articleUrl = decodeURIComponent(data.url);
                var indexOfTitle = [];
                var indexOfContent = [];
                // only match articles with not empty titles
                if(title != '') {
                  keywords.forEach(function(keyword) {
                    function getIndexByWord(word, text, caseSensitive) {
                      var wordLen = word.length;
                      if (wordLen === 0) {
                        return [];
                      }
                      var startPosition = 0, position = [], index = [];
                      if (!caseSensitive) {
                        text = text.toLowerCase();
                        word = word.toLowerCase();
                      }
                      while ((position = text.indexOf(word, startPosition)) > -1) {
                        index.push({position: position, word: word});
                        startPosition = position + wordLen;
                      }
                      return index;
                    }

                    indexOfTitle = indexOfTitle.concat(getIndexByWord(keyword, titleInLowerCase, false));
                    indexOfContent = indexOfContent.concat(getIndexByWord(keyword, contentInLowerCase, false));
                  });
                  if (indexOfTitle.length > 0 || indexOfContent.length > 0) {
                    isMatch = true;
                    hitCount = indexOfTitle.length + indexOfContent.length;
                  }
                }

                // show search results

                if (isMatch) {
                  // sort index by position of keyword

                  [indexOfTitle, indexOfContent].forEach(function (index) {
                    index.sort(function (itemLeft, itemRight) {
                      if (itemRight.position !== itemLeft.position) {
                        return itemRight.position - itemLeft.position;
                      } else {
                        return itemLeft.word.length - itemRight.word.length;
                      }
                    });
                  });

                  // merge hits into slices

                  function mergeIntoSlice(text, start, end, index) {
                    var item = index[index.length - 1];
                    var position = item.position;
                    var word = item.word;
                    var hits = [];
                    var searchTextCountInSlice = 0;
                    while (position + word.length <= end && index.length != 0) {
                      if (word === searchText) {
                        searchTextCountInSlice++;
                      }
                      hits.push({position: position, length: word.length});
                      var wordEnd = position + word.length;

                      // move to next position of hit

                      index.pop();
                      while (index.length != 0) {
                        item = index[index.length - 1];
                        position = item.position;
                        word = item.word;
                        if (wordEnd > position) {
                          index.pop();
                        } else {
                          break;
                        }
                      }
                    }
                    searchTextCount += searchTextCountInSlice;
                    return {
                      hits: hits,
                      start: start,
                      end: end,
                      searchTextCount: searchTextCountInSlice
                    };
                  }

                  var slicesOfTitle = [];
                  if (indexOfTitle.length != 0) {
                    slicesOfTitle.push(mergeIntoSlice(title, 0, title.length, indexOfTitle));
                  }

                  var slicesOfContent = [];
                  while (indexOfContent.length != 0) {
                    var item = indexOfContent[indexOfContent.length - 1];
                    var position = item.position;
                    var word = item.word;
                    // cut out 100 characters
                    var start = position - 20;
                    var end = position + 80;
                    if(start < 0){
                      start = 0;
                    }
                    if (end < position + word.length) {
                      end = position + word.length;
                    }
                    if(end > content.length){
                      end = content.length;
                    }
                    slicesOfContent.push(mergeIntoSlice(content, start, end, indexOfContent));
                  }

                  // sort slices in content by search text's count and hits' count

                  slicesOfContent.sort(function (sliceLeft, sliceRight) {
                    if (sliceLeft.searchTextCount !== sliceRight.searchTextCount) {
                      return sliceRight.searchTextCount - sliceLeft.searchTextCount;
                    } else if (sliceLeft.hits.length !== sliceRight.hits.length) {
                      return sliceRight.hits.length - sliceLeft.hits.length;
                    } else {
                      return sliceLeft.start - sliceRight.start;
                    }
                  });

                  // select top N slices in content

                  var upperBound = parseInt('1');
                  if (upperBound >= 0) {
                    slicesOfContent = slicesOfContent.slice(0, upperBound);
                  }

                  // highlight title and content

                  function highlightKeyword(text, slice) {
                    var result = '';
                    var prevEnd = slice.start;
                    slice.hits.forEach(function (hit) {
                      result += text.substring(prevEnd, hit.position);
                      var end = hit.position + hit.length;
                      result += '<b class="search-keyword">' + text.substring(hit.position, end) + '</b>';
                      prevEnd = end;
                    });
                    result += text.substring(prevEnd, slice.end);
                    return result;
                  }

                  var resultItem = '';

                  if (slicesOfTitle.length != 0) {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + highlightKeyword(title, slicesOfTitle[0]) + "</a>";
                  } else {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + title + "</a>";
                  }

                  slicesOfContent.forEach(function (slice) {
                    resultItem += "<a href='" + articleUrl + "'>" +
                      "<p class=\"search-result\">" + highlightKeyword(content, slice) +
                      "...</p>" + "</a>";
                  });

                  resultItem += "</li>";
                  resultItems.push({
                    item: resultItem,
                    searchTextCount: searchTextCount,
                    hitCount: hitCount,
                    id: resultItems.length
                  });
                }
              })
            };
            if (keywords.length === 1 && keywords[0] === "") {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-search fa-5x" /></div>'
            } else if (resultItems.length === 0) {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-frown-o fa-5x" /></div>'
            } else {
              resultItems.sort(function (resultLeft, resultRight) {
                if (resultLeft.searchTextCount !== resultRight.searchTextCount) {
                  return resultRight.searchTextCount - resultLeft.searchTextCount;
                } else if (resultLeft.hitCount !== resultRight.hitCount) {
                  return resultRight.hitCount - resultLeft.hitCount;
                } else {
                  return resultRight.id - resultLeft.id;
                }
              });
              var searchResultList = '<ul class=\"search-result-list\">';
              resultItems.forEach(function (result) {
                searchResultList += result.item;
              })
              searchResultList += "</ul>";
              resultContent.innerHTML = searchResultList;
            }
          }

          if ('auto' === 'auto') {
            input.addEventListener('input', inputEventFunction);
          } else {
            $('.search-icon').click(inputEventFunction);
            input.addEventListener('keypress', function (event) {
              if (event.keyCode === 13) {
                inputEventFunction();
              }
            });
          }

          // remove loading animation
          $(".local-search-pop-overlay").remove();
          $('body').css('overflow', '');

          proceedsearch();
        }
      });
    }

    // handle and trigger popup window;
    $('.popup-trigger').click(function(e) {
      e.stopPropagation();
      if (isfetched === false) {
        searchFunc(path, 'local-search-input', 'local-search-result');
      } else {
        proceedsearch();
      };
    });

    $('.popup-btn-close').click(onPopupClose);
    $('.popup').click(function(e){
      e.stopPropagation();
    });
    $(document).on('keyup', function (event) {
      var shouldDismissSearchPopup = event.which === 27 &&
        $('.search-popup').is(':visible');
      if (shouldDismissSearchPopup) {
        onPopupClose();
      }
    });
  </script>





  

  

  

  
  

  

  

  

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</html>
